Smoothing network



Dean 27, 1949 H. w. BODE SMOOTHING NETWORK Filed Feb. 23, 1944 VAX/S "F/l. TER [-76.3

COMPUTING SYSTEM FILTER F/6.3 1

INVENTOE W H. W 3005 A 7'TORNE V Patented Dec. 27, 1949 SMODTHING NETWORK Hendrik WI Bode;- New York; Nan, asslgnon to Bell Telephones Babel-arteries; Inporporated,--, New York, N. SE 1a:-corporation-ofiNewYork Appliiiation'February 23, 194 1; sieriarNog. 52:;51'0

(Clause-44) 3 Claims. 1 i This" invention relates to smoothing systems, and particularly to systems in which the variations in some" physical phenomenon are represented by; variations in a quantity of energy.

Inmany measunng'systems: the" steaditchan'ge in some natural: phenomenon is-measured, and; from this measurement: the rate of changesof: the phenomenon; is.= obtained; The? phenomenon. msasuredizmay beanyl 'maturabphenomenoh Which is ohanging'f at a; substantially. steady: rates. such as the unaccelerated motion oisa bbdmthe varias tion in the length, Weight, temperature or other propert of a substance; orsimilan change. The observations of' the phenomenon control the quantity of energy drawn from a suitable soufice} and, though the energy drawn from the source thus is proportional to; thechange in the phenomenon, this quantity of." energy will also have undesired perturbations due-- to observational errors and defects in: the: measuring: syst'enrr Iriaocordanc'e' withithe present invention; thequarr tit of energy proportionalitoithvrate o?change ofi'the-phenonrenonis supplied to a systom which smoothsthisquantity: oflenergm'tosreduce ithienmfv desired, perturbations and weights: the vaules; 01*

the energy: byfthe-methodr.ofg least: squares;

'liheeobjectof the; invention is a networlerorfila ten which modifies: thequantity of energy-town)? duce a-second; quantity of energy; proportional? to: the: rate-of; changefinr the firsttquantity; smooths;- the perturbations in this second quantitm cfenergy due-to observationallina-ccuracies andmndesiiedi variations; and weights the. resultsioii'thep observations.

A feature ofthe invention isia nettvork which weigiitsl successive values of the second quantity of: energy: in. accordance with afunction-' whichva ies from: zero-tat the terminus of a past tiin'e interval through a?maximum=to zero'at the pres ent tiinel: 'I l'ie variatioi'i' -in the r-motion maybe approxi-mately arabolic.

In the drawings Figs.rishowstheageometrieal" relationships for sr cynical-physicalphenomenon;

Fig. 2 diagrammaticall shows; a system 113:7 computing certainzvaluesiromobsetirationsibfithe phenomenon ofiEis: 1

Rig: 'efisshowsi a network: orfilter used-linifigi and.

Eig. 4 showsranlamplifienused-in Fig. 1 shows the}: geometrical rehutionshibs; foe aatyniaalsphysical;phenomenon; the mosriementrini thrfiesdimensionalasnace; a-.-targe t, an: aerial-vehicla an air-plane, airshipv etc;-

obhtfiil-lblisly liiider observation from-some O'BS'Gfiration station- 63 The angular height E0 of the mge t a bbvera horizontal planegand: the: arigle o'fi azimuth- Ao of the: target from some selected dismotion ON are" continuously nea'sured by theusua-l instruments; such ais al thebdolitaohrfidiol location device; The slantf distance 02110;: desigi 1i); icontinuously measure-(1' by? khbw'ii -,"SuO11 iaIS a rfa'nge finder, radio apparatus; or By triangulation For unaccelerat'ed motion 0T the targetthe rate's of chan ge -of the' polarcoon dlii'atbs A0} m, or the tar et will? notmsuailly be constanl and canrrot easilybe smoothefi.-- me oise coordinates E6, A0; we thus are: first trans: formed: intoi a r system of; rectilinear coordinates- :a y l, 'qq inswhich the rates of; change of. theme ondinatesrtheoretioally are constant; A

t m 2a source of'voltagel such as a batfiery generator or; rectified andffiltered power S1115? ply withthenegja'tive polegrouhdedj is connected across; the potentiometer winding; 2; Another source df voltage- 3; with" the ositive little" eronndedi connected a'crossf the potentiome winding 4. The brushes" 5; 5 are insulated eacii other" am riomground; and are rotated n? a" clockwise. direction, either manually. or automatic devices; in accordance; with the observed values, to select voltagsWith resiie'cfitif ground, respectively positive and negative;- propbrt'ional to To, the slant distance to the'=-targeti I The' bru'sfi S' is' OnnectedtQ a tap" 8 inf th stentiometer wlndingl, and the brushfifi'sicoin nected to a diametrically opposite tap 9" hr the earn-c winding: he equidistanti intermediate points I0; l l o'f the potentiometer winding 'r'aire grounded. fIThe potentiometer winding 1, as sche matiirally 'showri. has tin-approximatelysinusoidal variation'inresistaricei 'Ilie brushes I21 I3? I! are insulated fromeach other andiroin ground and arerotatedj either'manuall'y or lay-known automatic devices, in accordance withthe observed. values'oftlieangular'heightEb. I

ll nom Eig 1, uou= rac sin; Eohthusyttlfefibiffishl IE5 is i'iltcloclitvlseifroinzerofahglelatTth ,lidifitilnn tosele'ct a'voltaeewitnrespecttoigroundnproiion; tional tor sin Ent FronrFfg; 1; the horizontal" distance do=ro cos E0"? thus the brushes l3, l4 larelooated degrees from the: brush: lZ-;-; nderespective1y selectr posia tive and negative voltages with respect to ground proportional to To cos-Ea;

The brushes l3, M are respectively connected Thertarget isimoving QIQHg;8,Z-OOHE$JTQT"WHQJ$ 56' to diametrically opposite taps l5, IS in the ti-ometer winding ll. The equidistant, intermediate points l8, 19 of the winding l1 are grounded. The potentiometer winding I! also has an approximately sinusoidal variation of resistance.

From Fig. 1, xo=d sin A0 and yo=do cos A0. The brushes 20, 2! are rotated, either manually or by known automatic devices, clockwise from zero angle at point [8 in accordance with the observed values of the angle A0, to select voltages with respect to ground respectively proportional to .730 and ya.

The potentiometer windings 2, 4, I, Il may be in the form of cards of suitable material having one straight edge, the other edge being cut to the proper shape, closely and evenly wound with resistance wire which is cleaned along the straight edge to make a good contact with the brushes.

The voltages proportional to the observed values, :00, yo, no, of the rectangular coordinates of the position of the target T0, are supplied by connections 22, 23, 24 to any suitable utilization means, such as a known computing system 25. As many such computing systems are known, such as the systems shown in United States Patent 2,408.081, September 24, 1946, C. A. Lovell et 2.1., and United States application Serial N0. 443,920, filed-May 21, 1942, by W. H. Boghosian et al., both assigned to the assignee of the present application, a detailed description of such a system in the present application is not necessary.

Assume that, since the time to in the past the target has been continuously observed, and has moved during a time interval t1 ending at the present time t at the component velocities $0, yo, 170,

where me, etc. represent the time derivatives of the movement. The true course of the target is given by mo-i-scotl, yu-l-yoti, vo+vot1, the true coordinates of the position of the target at the time t are :r, y, '0, and the observed coordinates are $1,111, in. The observed results are to be averaged over a time interval T, that is, over the past time interval from tT to t, the present time. Of course, as time passes and t assumes new values, the past time interval tT, though of constant length, will move with t to include new values of the data, and exclude old values.

The observers will not be able to follow the movement of the target with perfect accuracy. Thus the observed results will have observational perturbations which should be averaged. The errors may be assumed to be random in distribution, thus the integral of the squares of the vector errors (ac-r1), (yy1), (2-21) should be a minimum.

Thus, the integral, over the past time interval from the present time t to the past time t- T,

in which 151 is the variable of integration, should be a minimum.

For convenience, this discussion may be limited to the ac coordinate, as similar results will be obtained for y and v. V

For the target course assumed, Equation 1 becomes I fl aae-arara 2) and this integral is to be a minimum, that is l l b$ bzt Thus As to is of present interest, we may be eliminated, and as the integration in Equations 1, 2, 3, 4 is performed from the past time t-T to the present time t, whereas it is more convenient to integrate from the present time if over the past time interval T, for convenience, t1 may be replaced by u=t1t+T where the quantity u represents time H extending into the past from the time of observation t.

Thus, from Equations 3 and 4 T .eg may. (5)

, By integration by (parts, Equation 5 becomes 6 T flie -T 1; x (u uT) du (6) Equation 6 shows the type of weighted average e-en Equation 7 when plotted against the ratio t/T is a section of a parabola, and the weighting function (u uT) of Equation 6 when plotted against u/T is also a section of a parabola.

The values of 0:1 must h diiferentiated to produce the value x1 and then smoothed by some network. The weighting -function (u uT) may be interpreted as the response of the smoothin network only to an impulse of unit area and negligible width, or as the response of the differentiat-or and network to a unit step function, that 50 is, a suddenly applied voltage of constant unit amplitude.

In Equation 6, it is the time derivative 1:1 of the observed value an which is to be averaged and weighted by the network or filter to give the smoothed values :61. The connections 22, 23, 24, respectively, apply voltages proportional to :m, 111, 7.71 to the capacitors 28, 21, 28 connected to the input circuits of the amplifiers 29, 30, 3| which 5 may be of the type disclosed in U. S. Patent 2,401,779, June 11, 1946, K. D. Swartzel, Jr., as shown in Fig. 4. The currents from the capacitors 26, 21, 28 are respectively proportional to the time rates of change of the applied voltages, 65 that is,t0 $1,111,171.

In Fig. 4, the input terminal 32 is connected through resistor 34 to the'control electrode of vacuum tube 35. The grounded input terminal 33 is connected to the cathode of vacuum tube 35.

The output circuit of vacuum tube 35 is connected by any suitable interstage coupling network, such as the network shown in U. S. Patent 1,751,527, March 25, 1930, H. Nyquist, to the input circuit of vacuum tube 36. The output cir- 75 cult of vacuum tube 3-6 is similarly connected to and . 5 the input circuit oi' vacuum tubetl. Bower is suppliediin the usualimanner to the anodes, screen grids and heaters (not shown) Power is supplied by asnitab'le source 39 through resistor 38 to the anode. circuit of vacuum tube 31'. A suitable source'of power 40 has the negative pole connected to the cathode of vacuum tube 31 .and thepositive pole grounded. The potentials of the sources 39, 4D and the resistance of resistor 38 are selected so that, in the absence of 'asignal emltage applied to the amplifier, thesources .39, 40,. and the resistances of the resistor 33 and the anode-cathode path of vacuum tube '3] iorm a balanced bridge, thus no voltageisapplied to the output terminals 41, 4'2. Whena signal voltage is applied to the amplifier and the. amplified voltage .is applied to the control electrode of vacuum tube 31,, the anode-cathode resistance of vacuum tube 3! is changed, un-

lbalancing the bridge and producing. a voltage across the output terminals. 4 42. As the amplifier has an odd number of stages, the voltage producedacrossthe output terminals- 4 I, 42 is opposite in polarity to the voltage applied to the input terminals 32, J33.

The winding of potentiometer 43 may be connected across theoutput terminals 41,-4.2, and the brush connected through resistor 44 to supply reverse feedback to the. input circuit of the amplifier. some cases potentiometer 43 may be omitted .and the feedback connection may be made directly to the anode .of vacuum tube 31 If. .e be the voltage applied .to capacitor 26, C the capacitance of capacitor 26, r the resistance of resistor 34, R the resistance of resistor 4.4, ,u

the voltage amplification of the amplifier, en the voltage developed across the winding of potentiometer 43:, 5c the traction of this voltage selected by the brush and p the usual operational symbol for a tune derivative, it may be shown that 1 I e v pe (9) 1 +(r+ )cp With modern vacuum tubes, F mayeasily be made large, thus n the targetwere moving in a straight line .at

constant speed, and the observed values were ac- I curate, then 60 would be a linear function of time, the time derivatives of the movements in the three coordinates would be constant and the voltages supplied the networks 45, 4B, 41 would be proportional to $0, ya, '00. In general, the observed values mi, 2 11, m are not quite accurate, thus e is not quite a linear function of time, and the operator RC P introduces some smoothing of the derivatives of the observed values. This initial smoothing may be taken into account in the design of the smoothing network. The voltages supplied to the smoothing networks thus are partially smoothed values of an, m, 01. The combinations of capacitor 26, amplifier 29, filter 45, potentiometer E9; capacitor 21, amplifier 30, filter 46, potentiomer 6|; capacitor 28, amplifier 3|, filter 41, potentiometer 62, respectively form transmission The transient response corresponding to a unit impulse is the derivative of the above, or

In a general network, the as may be real or complex. In a smoothing circuit, it is seldom possible to use inductors, as the values of inductance or efiect'ive resistance often are impractical, thus capacitor-resistor networks are of principal interest. In networks of this type, the as in Equations 11 and 12 are all positive real quantities.

In Equations 11 and 12 it is assumed that the roots of the characteristic determinant of the system are :all distinct. If some of the roots are equal, the expression takes a diiierent form. For example, if there are n roots all equal to a, Equation 11 is replaced by vNo exact technique is known for establishing the values of the As and as in Equations 11,, 12, 13. One method is to first assume coincident roots, and, using the desired conditions for settling time, determine a from Equation 13. Then, by polynominal approximations, the As may be determined.

The coincident root solution frequently leads to extreme values of the elements in the network. Non-coincident roots may then be chosen, more or less centered about the coincident roots. The selected values of a are introduced into Equation 11., and values of the As chosen to "fit :a desired number, of points on the parabolic characteristic. Frequently, these first values of a and A willproduce a characteristic with a rather long and slowly varying tail, having current values :which are .too small to materially aid the smoothing, but too large for desired accuracy, thus requiring a long wait before the values may be used in the prediction. The characteristic may then be shortened by modifying the values of the Afs and contracting the characteristic to bring the tail within the desired settling time, thus producing :a characteristic which, outside the weighting interval, is zero and stays zero with considerable accuracy.

In a typical network, Equation 12 had the following values:

f (t) 39248- mama- 20.394e- Meme- 14 0 Equation 11 may also be written in the following form:

f( o+ 1+ 2+ n) 1( 2( Physically, this characteristic may be realized initially by a lattice or bridge type network, having "positive terms in the series arms, and negative terms in the lattice arms, depending upon the signs of the As. Thus, the term to (Ao+A1+ of Equation 15 corresponds to a simple resistance, represented by the resistor 50 of Fig. 3. The terms of the form Aid-e are simple combinations of resistance in series with capacitance, and, if positive, are represented by 75 combinations in the series arms, such as resistor team .51 and capacitor 52, and, if negative, by combinations in the lattice arms, such as re'sistor53 and capacitor 54, or resistor 55 and capacitor 56. The lattice is completed by identical series and lattice arms. 1

After the design of the lattice network has been completed, the lattice network may be transformed into an unbalanced network, such as a bridged-T network, by the methods set forth in my prior Patents 1,944,209, January 23, 1934; 2,029,698, February 4, 1936; and 2,058,210, October 20, 1936.

The output circuits of the filters or networks 45, 46, 47 are respectively connected, if desired, through amplifiers of the type shown in Fig. 4,

to the windings of potentiometers 69, 6!, 62. The

brushes of potentiometers 60, 6i, 62 are adjusted manually or by known automatic means, to select voltages proportional to any desired time interval, such as the time of flight At, of a shell traveling from the point of observation to the target and thus to supply to the computer 25 voltages proportional to the smoothed and weighted values of the predicted increments mint, ymt, and 121M,

in the rectangular coordinates of the position of the target.

While, for convenience, the invention has been disclosed as embodied in a typical electrical system, the invention is not limited to the specific system shown nor to electrical systems. The analogies between electrical and mechanical systems are well known, and the present invention may easily be applied to mechanical systems to determine the constants of a mechanical transmission which will produce a smoothed and weighted average of a mechanical movement.

What is claimed is:

1. Means for producing a current proportional to a smoothed weighted average of the rate of change of a voltage nominally varying at a con-- stant rate but having undesired perturbation-s,

comprising a capacitor connected to the source of said voltage, a feedback amplifier connected to have low impedance input and output circuits, a resistor connecting said capacitor to said input circuit, and a network connected to said output:

circuit comprising series and shunt arms formed of dissipative and reactive elements so proportioned that the indicial admittance of said means varies from zero through a maximum to zero in stant rate but having undesired perturbations, i

comprising a capacitor connected to the source in which the As are respectively the admittances of the resistive elements of the arms, and the we are respectively the time constants of the arms.

3. Means for producing a current proportional to a smoothed weighted average of the rate of change of a voltage nominally varying at a constant rate but having undesired perturbations, comprising a capacitor connected to the source of said voltage, a feedback amplifier connected to have low impedance input and output circuits, a resistor connecting said capacitor to the input circuit, and a network connected to the output circuit having electrical characteristics equivalent to a lattice network with series arms formed of a resistor in parallel relationship with the serial combination of a resistor and a capacitor, and lattice arms formed of the serial combination of a resistor and a capacitor in parallel relationship with another serial combination of a resistor and a. capacitor, said resistors and capacitors being so proportioned that the indicial admittance of said means varies from zero, through a maximum to zero in a predetermined time interval.

HENDRIK W. BODE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

